NR-U’s definition of success for LBT needs to be realigned with 802.11 & European rules

1. NR-U’s definition of success for LBT needs to be realigned with 802.11 & European rules • 1 July 2019 Authors: Andrew Myles, Cisco

2. NR-U’s definition of success for LBT needs to be realigned with 802.11 and European rules • Executive Summary • European rules & IEEE 802.11 base Contention Window (CW) updates in LBT on the success of transmissions at the start of a COT … • … because only the success (or lack thereof) at the start of COT is relevant to collision detection for LBT • Whereas, LAA/NR-U bases Contention Window(CW) updates on information later in the COT, contrary to the European rules • It is time to align NR-U to focus success on start of transmission to enable compliance & fairness Cisco

3. European rules & IEEE 802.11 base CW updates in LBT on the success of tx’s at the start of a COT … • The European 5 GHz rules (as currently documented in EN 301 893 v2.1.1) for updating the LBT Contention Window (CW) require … • … atleast one transmission that started at the beginning of the Channel Occupancyneeds to be successful before CW can be reset, otherwise CW is doubled • The CW update rules in for EDCA based access in IEEE 802.11 are consistent with these European rules • IEEE 802.11 generally focuses on start of a TXOP to determine success or otherwise for the purposes of CW updates • There are some interesting exceptions in the IEEE 802.11 standard, such as broadcast frames and delayed block acks • However, they generally do not matter in practice; see BRAN(19)102015 for more details Andrew Myles, Cisco

4. … because only the success (or lack thereof) at the start of COT is relevant to collision detection for LBT Errors at the start of a COT indicate a collision for LBT; no errors at the start indicate a lack of a collision COT/TXOP 1 Errors later in a COT are not indicative of a collision Collision COT/TXOP 2 Note: An implicit assumption is that the start of transmissions align, which is true if one assumes successful use of hidden station mitigation mechanisms Andrew Myles, Cisco

5. Whereas, LAA/NR-U bases CW updates on information later in COT, contrary to the European rules • It is understood that LAA/NR-U does not focus its evaluation of success at the start, possibly making it non-compliant with the European rules • Rather, it evaluates the fraction of the COT that was rx’ed correctly to determine success, which may not include the start of the COT • The situation is even worse if there is significant use of reservation signals at the start of a COT because this period is not evaluated at all • The result is unfair access to LAA/NR-U, relative to Wi-Fi! Wi-Fi detects collision If 80% correct reception is threshold for success then this case might not be detected as a collision by LAA/NR-U; it should be! Wi-Fi TXOP Reservation signal NR-U COT Andrew Myles, Cisco

6. It is time to align NR-U to focus success on start of transmission to enable compliance & fairness • Now would be a great time to fix NR-U so that collision detection focuses on the start of the COT • The fix may require significant changes to the NR-U specification • It may be less important to fix LAA, depending on one’s view of its market importance • A fix is important to: • Avoid future compliance issues for NR-U in Europe ... • … and more importantly to promote fair access in a shared environment by aligning NR-U with the spirit of LBT based coexistence Andrew Myles, Cisco